ICMP Type 3 – Destination Unreachable

The Type 3 is given then a destination cannot be found or is simply unreachable by the sender.

However, the ICMP Type 3 gives a bit more details by adding a code to the message:

• Code 0—network unreachable
• Code 1—host unreachable
• Code 2—protocol unreachable
• Code 3—port unreachable

Therefore, combining the ICMP Type 3 message with a unique code gives you, the network professional, a better idea of the error on the network.